The phenomenon of superconductivity was first discovered in the early 1900s. A superconducting material conducts current with zero energy loss and expels magnetic field (like a perfect diamagnetic material) when cooled below the transition temperature. Until the mid 1980s, all known superconducting materials were metallic compounds such as mercury (Hg), lead (Pb), and niobium-tin (Nb.sub.3 Sn). In general, these materials become superconducting at temperatures below 40 degrees Kelvin, depending on the material, by undergoing a transition from to the normal, resistive state to the superconducting state. The transition temperature (T.sub.c) is a material specific temperature. For any material in the superconducting state at a given temperature and applied magnetic field, there is a maximum current density that the material is able to conduct without developing resistance. The critical current density (J.sub.C) is also one of the factors that limits the maximum magnetic field H, at which a superconductor can remain in the superconducting state. As the externally applied magnetic field (H) increases, the critical current density J.sub.C (T, H) decreases. Above some critical field, H.sub.c, the material can not support any current in the superconducting state and undergoes transition to the normal state. Both H.sub.c (T) and J.sub.C (T) increase when decreasing the cooling temperature of the superconductor.
Depending on certain magnetization properties, a superconducting material can be characterized as a type I superconductor or a type II superconductor. When increasing the applied current or magnetic field, or raising the temperature above T.sub.C, type I superconductors undergo a direct transition from the perfectly diamagnetic state (i.e., the Meissner state) to the normal state. Type II superconductors, however, first develop a "mixed (vortex) state," wherein the applied magnetic field penetrates the superconducting material above the lower critical field (H.sub.c1), and then the material undergoes the transition to the normal state above the upper critical field (H.sub.c2). When the magnetic filed is raised above H.sub.c1, it becomes energetically more favorable to admit into the material individual flux quanta in vortices than to maintain the Meissner state with the total flux exclusion. The vortices are distributed over the superconducting material to achieve an energetic minimum. When a transport current passes through the superconductor in the mixed state, the Lorentz force acts on the vortices. At the same time, chemical and physical defects in the superconducting material may keep the vortices "pinned" at the location of the defect. If the Lorentz force (which is proportional to the current density) exceeds the pinning forces, the vortices start to move and dissipate heat, which leads to resistivity.
In the mid 1980s, first high temperature superconductors (HTS) based on oxides of copper compounds were discovered. Some of these materials displayed superconductivity above liquid nitrogen temperature (T.sub.c &gt;77 K) allowing dramatically more practical and economical cooling. For example, the HTS materials are compounds of RE.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-.delta. wherein RE=Y, Nd, La, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu; the Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.x, (Bi, Pb).sub.2 Sr.sub.2 CaCu.sub.2 O.sub.x, Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.x and (Bi, Pb).sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.x compounds; the Tl.sub.2 Ca.sub.1.5 BaCu.sub.2 O.sub.x or Tl.sub.2 Ca.sub.2 Ba.sub.2 Cu.sub.3 O.sub.x compounds and compounds involving substitution such as the Nd.sub.1+x Ba.sub.2-x Cu.sub.3 O.sub.x compounds. These copper oxide superconductors are type II superconductors.
Several researchers have focussed on introducing controlled defects into the HTS materials to increase the pinning forces. These defects reduce the movement of the fluxoids and permit high critical currents even at relatively high temperatures and high magnetic fields. Magnetic field that penetrates the superconducting material may also lead to "trapped" magnetic field. The trapped field can be pinned in place even when there is no supporting external magnetic field. An ingot of superconducting material with trapped magnetic field, not supported by another magnet, is called a trapped field magnet, and is similar in some ways to a permanent magnet.
When the externally applied magnetic field (or the applied current) is removed, the trapped magnetic fields decay over time, which is called flux creep. Flux creep tends to stabilize the flux distribution in the superconducting material by relieving the magnetic pressure. Flux creep, which decays approximately logarithmically over time, is a measure of loss of the trapped magnetic field. The magnetic flux density is supported by the pinning force and is related to the current density by Ampere's law. As the current density is increased toward the critical current density J.sub.C (T), flux creep increases.
As mentioned above, particular defects increase pinning of the fluxoids. The optimal defect diameter is determined by two parameters of the superconducting material. The first parameter is the magnetic field penetration depth, which determines how far from the defect the magnetic field may penetrate into the superconducting material. The second parameter is the coherence length, which determines how far into the superconductor, from the defect, the vortex current builds up.
In 1989, vanDover et al. reported that J.sub.C and H.sub.irr improve when YBa.sub.2 Cu.sub.3 O.sub.7 single-crystals are irradiated either with thermal or fast neutrons (See R. B. van Dover et al., "Critical Currents Near 10.sup.6 A/cm.sup.2 in Neutron-irradiated Single-Crystal YBa.sub.2 Cu.sub.3 O.sub.7 ", Nature, vol. 342, pp. 55-57, 1989.).
Civali et al. reported creation of columnar defects in YBa.sub.2 Cu.sub.3 O.sub.7 crystals by 580-MeV Sn-ion irradiation. The columnar defects allow the field to pass through the material and also serve as pinning centers with attractive potentials that reduce the flux creep. (See Civali et al., "Vortex Confinement by Columnar Defects in Yba.sub.2 Cu.sub.3 O.sub.7 Crystals: Enhanced Pinning at High Fields and Temperatures", Phys. Rev. Lett., vol. 67, p. 648, 1991).
Weinstein et al. reported increase in the critical current density of the YBCO material exposed to high-energy light-ion irradiation. (See R. Weinstein et al., "Materials, Characterization and Applications for High T.sub.C Superconducting Permanent Magnets", Applied Superconductivity, Vol.1, pg. 1145-1155, Pergammon Press, 1993).
Fleischer et al. reported some increase in J.sub.C and H.sub.irr in polycrystalline bulk HTS materials or films doped with a mixture of uranium-238 and uranium-235 and subsequently irradiated with thermal neutrons to cause fission. (See Fleischer et al., "Increased Flux Pinning upon Thermal-Neutron Irradiation of Uranium-doped Yba.sub.2 Cu.sub.3 O.sub.7 ", Phys. Rev. B, Vol. 40, pp. 2163-2169, 1989; Luborsky et al., "Critical Currents after Thermal Neutron Irradiation of Uranium Doped Superconductors", J. Mater. Res., Vol. 6, pp. 28-35, 1991; Fleischer et al., "Increased Flux Pining Upon Thermal-Neutron Irradiation of Uranium-Doped Yba.sub.2 Cu.sub.3 O.sub.7 ", Gen. Electric Tech. Report #89CRD047, April 1989; and U.S. Pat. No. 4,996,192)
Safar et al. reported improved J.sub.C and H.sub.irr in the HTS bismuth materials irradiated with high-energy protons (800 MeV). (See Safar et al. Appl. Phys. Lett. vol. 67, p. 130, 1995)
In general, there is a need for a high T.sub.c superconducting material with T.sub.c above 77 K and high values of J.sub.C and H.sub.irr, which can be economically produced in uniform bulk quantities, or in form of thick or thin films, and which are suitable for different superconducting applications.